KR20180024317A - Bi-directional dc-ac invertor - Google Patents

Abstract

The present invention relates to a bidirectional DC-AC inverter. The bidirectional DC-AC inverter according to an embodiment of the present invention may include a bidirectional DC-AC inverter part including a three-phase three-level T type structure to be coupled with a three-phase four-wire system power source. According to the present invention, three-level switching of the bidirectional DC-AC inverter reduces a harmonic component of an output current and reduces the size of an output AC filter.

Description

Bidirectional DC-AC inverter {BI-DIRECTIONAL DC-AC INVERTOR}

The present invention relates to a bidirectional DC-AC inverter, and more particularly, to a bidirectional DC-AC inverter used in an energy storage system.

The Smart Grid is a next-generation intelligent power grid that exchanges information in real time between power suppliers and consumers in order to optimize energy efficiency.

In recent years, photovoltaic power generation has been mainly used as a means for utilizing renewable energy. As a result, the power generated from the photovoltaic power generation is stored in a separate energy storage device, thereby reducing the utilization rate of the grid power and reducing the use of the grid power. .

As described above, the bi-directional inverter is provided inside the bi-directional inverter in order to store the power generated by the photovoltaic power generation in the energy storage device including the battery. In the conventional bidirectional inverter, the internal voltage of the switching device is higher than the DC- A device having an internal pressure is required. There is a problem in that it is difficult to use a high voltage such as a rise in the unit price of the switching element, and the capacity of the output AC filter becomes large.

Korean Patent No. 10-1595060 (Feb.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a three-phase three-level bidirectional DC-AC inverter that can be connected to a three-phase four-wire system power supply.

The bidirectional DC-AC inverter according to an embodiment of the present invention may include a bidirectional DC-AC inverter unit including a three-phase three-level T-type structure for coupling with a three-phase four-wire system power source.

의 3레벨 상태가 가능할 수 있다.At this time, the bidirectional DC-AC inverter unit performs a clamping operation including a neutral point of the output voltage

A three-level state of < / RTI >

The PLL controller may further comprise a proportional-integral controller for setting the d-axis value of the PLL to be zero so that the PLL is configured such that the angle at which the d-axis value of the synchronous coordinate system becomes zero is determined as the reference angle.

Here, the proportional-integral controller can detect the phase of the system by forward-compensating the expected system frequency.

The voltage controller may further include a voltage controller to which an error occurring at the difference between the DC link command voltage and the actual DC link voltage is input and outputs a maximum value command of the system current.

At this time, the voltage controller can generate a sinusoidal current command synchronized with the three-phase system by multiplying the phase extracted through the three-phase PLL control by the maximum value command of the system current.

The current controller may further include a current controller that receives an error generated from the difference between the three-phase sinusoidal current command and the actual three-phase current, and outputs the duty command so that the direct current link follows the command voltage.

Here, the voltage controller may be a proportional-integral controller.

According to the present invention, the three-level switching of the bidirectional DC-AC inverter reduces the harmonic component of the output current and reduces the size of the output AC filter.

Further, as compared with the conventional three-level NPC type inverter, it is possible to use a switch element in which the number of two or more diodes can be reduced and the voltage rating of the neutral point switch is lowered by cutting each leg switch rating, The common emitter configuration can reduce the number of power supply insulation of the driving circuit to one.

1 is a diagram illustrating a bidirectional DC-AC inverter according to an embodiment of the present invention.
2 is a block diagram of a PLL control block of a bidirectional DC-AC inverter according to an embodiment of the present invention.
3 is a block diagram illustrating a voltage-current controller in a grid connection of a bidirectional DC-AC inverter according to an embodiment of the present invention.
4 is a block diagram illustrating a capacitor voltage imbalance compensation algorithm of a bidirectional DC-AC inverter according to an embodiment of the present invention.
5 is a diagram illustrating a voltage waveform of a capacitor voltage according to whether a capacitor voltage imbalance compensation algorithm of a bidirectional DC-AC inverter according to an embodiment of the present invention is applied.

Preferred embodiments of the present invention will be described more specifically with reference to the accompanying drawings.

FIG. 1 is a diagram illustrating a bidirectional DC-AC inverter according to an embodiment of the present invention, and FIG. 2 is a block diagram of a PLL control block of a bidirectional DC-AC inverter according to an embodiment of the present invention.

의 3레벨 상태가 가능하다.Referring to FIG. 1, a bidirectional DC-AC inverter according to an embodiment of the present invention is a three-level T-type inverter composed of a three-phase four-wire system. At this time, through the clamping included in the neutral point of the output voltage

3-level state of the < / RTI >

는 계통의 a상, b상, c상의 출력 전압이고, abc-dq로 변환하는 동기 좌표계 변환 합수는

로 표현한다. 이때, 동기 좌표계의 d축 값이 0이 되는 각을 기준각으로 결정하게 PLL을 구성할 수 있다. 그리고 d축 값이 0이 되도록 비례-적분 제어기를 설계할 수 있다. 또한, 예상되는 계통 주파수

를 전향 보상하여계통 주파수가 자주 변동되더라도 추종 오차 없이 계통의 위상을 검출할 수 있다.The three-phase PLL control of the bidirectional DC-AC inverter as described above will be described with reference to Fig. first,

Is the output voltage of the phase a, phase b and phase c of the system, and the synchronous coordinate system conversion sum to be converted into abc-dq is

. At this time, the PLL can be configured to determine the angle at which the d-axis value of the synchronous coordinate system becomes 0 as the reference angle. And the proportional-integral controller can be designed so that the d-axis value becomes zero. Also, the expected grid frequency

So that the phase of the system can be detected without a tracking error even if the system frequency changes frequently.

3 is a block diagram illustrating a voltage-current controller in a grid connection of a bidirectional DC-AC inverter according to an embodiment of the present invention.

Errors arising from the difference between the DC link command voltage and the actual DC link voltage are input to the voltage controller. To this end, the bidirectional DC-AC inverter may further include a voltage controller in this embodiment. The voltage controller outputs the maximum value command of the grid current. The sine wave current command synchronized with the three-phase system can be generated by multiplying the phase extracted by the three-phase PLL control by the maximum value of the system current.

The error caused by the difference between the three-phase sinusoidal current command and the actual three-phase current is input to the current control circuit, and the duty command can be output so that the DC link can follow the command voltage.

In this embodiment, the voltage controller can be designed as a proportional integral controller that is easy to implement. Because the frequency of the output current is the system frequency and there is little change, the current controller can be designed with a proportional resonant controller with a large gain at the frequency. Here, the transfer function according to the ideal case of the proportional-resonance controller is expressed by Equation (1).

[Equation 1]

는 출력 전류의 주파수이므로, 계통 주파수로 설정한다. 따라서 계통 주파수에서 큰 이득 값을 얻을 수 있지만, 계통 주파수로 선정된 공진주파수와 달라지면 이득 값이 현저히 낮아질 수 있다. 그리고 공진주파수에서의 아주 작은 변화에도 민감하게 반응하여 제어의 어려움이 따를 수 있다. 전류제어기의 안정성을 위해 좁은 공진 대역폭을 대역 통과 성분으로 추가하여 이득 값이 조금 줄어들더라도 공진주파수의 주변 대역폭에서도 큰 이득 값을 가지도록 전류제어기를 설계할 수 있다.At this time, the resonance frequency used in the transfer function of the proportional-

Is the frequency of the output current, it is set to the system frequency. Therefore, a large gain value can be obtained at the system frequency, but the gain value can be significantly lowered when it is different from the resonance frequency selected at the system frequency. It is also sensitive to very small changes in the resonant frequency and can be subject to difficulties in control. For stability of the current controller, a narrow resonance bandwidth is added as a band pass component, so that the current controller can be designed to have a large gain value even in the peripheral bandwidth of the resonance frequency, even if the gain value is slightly reduced.

The transfer function of the current controller to which the bandpass component is added is expressed by Equation (2).

&Quot; (2) "

와

는 각각 비례 이득 및 공진 이득을 나타낸다.In the transfer function of Equation (2)

Wow

Represent the proportional gain and the resonance gain, respectively.

When controlling a grid-connected bidirectional DC-AC inverter, the grid voltage can also be measured and added to the duty command value as a forward compensation component. In addition, as the forward compensation component is added, the characteristic of tracking the command value of the output current can be improved.

FIG. 4 is a block diagram illustrating a capacitor voltage imbalance compensation algorithm of a bidirectional DC-AC inverter according to an exemplary embodiment of the present invention, and FIG. 5 is a block diagram illustrating a capacitor voltage imbalance compensation algorithm of a bidirectional DC- The voltage waveform of the capacitor voltage according to the presence or absence of application of the capacitor.

Capacitor voltage imbalance can be caused by a variety of causes, which can be most prominent in a 3-phase 4-wire system power supply if the load is unbalanced or non-linear. In the case of an unbalanced load or a nonlinear load, the current flows to the neutral line, and the current flows into the neutral line between the divided capacitors of the DC link. The current flowing from the neutral line to the neutral point of the capacitor in this way may cause unbalance of the capacitor voltage.

When the DC link capacitor voltage is unbalanced, the output current is outputted in a distorted form rather than a sinusoidal wave, and a positive current or a negative current is not normally output due to the voltage of a small one of the two capacitors of the DC link. This causes voltage and current stress in the system as well as the bidirectional DC-AC inverter.

In this embodiment, to compensate for the DC link capacitor voltage imbalance, the unbalance of the capacitor voltage can be compensated by varying the modulation index (Mi) according to the degree of unbalance of the capacitor voltage.

The conventional capacitor voltage uncompensation compensation algorithm is a method in which the easiest and basic algorithm is applied to the offset voltage of the duty command through the proportional-integral controller by the difference of the two capacitor voltages. Such a conventional algorithm has a structure in which, when an unbalance occurs in the capacitor voltage, more energy is consumed on the higher side of the capacitor voltage. For example, when the voltage of the upper capacitor is larger, a positive offset voltage is added to the duty command to increase the time for outputting the positive duty command, thereby consuming more energy.

가 하단 캐패시터의 전압

보다 크면,

의 값은 0.5보다 큰 값을 갖는다. 여기에, 2를 곱하면, 변조지수가 1보다 조금 큰 값을 갖는다. 그 반대의 경우에는 변조지수는 1보다 조금 작은 값을 갖는다. 이러한 변조지수를 이용하여 캐패시터 전압의 불평형을 보상할 수 있다.In this embodiment, the capacitor voltage imbalance compensation algorithm can increase the modulation index without outputting the offset voltage, so that a larger voltage can be output to increase the energy consumption. For example, the voltage of the upper capacitor

The voltage of the lower capacitor

Lt; / RTI >

Lt; RTI ID = 0.0 > 0.5. ≪ / RTI > When multiplying by 2, the modulation index has a value slightly larger than one. In the opposite case, the modulation index is slightly smaller than 1. This modulation index can be used to compensate for unbalance in the capacitor voltage.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It should be understood that the scope of the present invention is to be understood as the scope of the following claims and their equivalents.

Claims (8)

Way bidirectional DC-AC inverter comprising a three-phase three-level T-type structure for coupling to a three-phase four-wire system power supply. The method according to claim 1,
The bidirectional DC-AC inverter unit is connected to the power source through clamping included in the neutral point of the output voltage

Bidirectional DC-AC interbars capable of three-level state. The method according to claim 1,
Wherein the PLL is configured such that the three-phase PLL control determines an angle at which the d-axis value of the synchronous coordinate system becomes zero as a reference angle, and the d-axis value becomes zero. The method of claim 3,
Wherein the proportional-integral controller detects the phase of the system by forward-compensating the expected systematic frequency. The method of claim 3,
The bidirectional DC-AC inverter according to claim 1, further comprising a voltage controller for receiving an error occurring at a DC link command voltage difference and an actual DC link voltage difference, and for outputting a maximum value command of the system current. The method of claim 5,
Wherein the voltage controller multiplies the phase extracted by the three-phase PLL control by a maximum value command of the system current to generate a sinusoidal current command synchronized with the three-phase system. The method of claim 6,
Further comprising a current controller to which an error caused by the difference between the three-phase sinusoidal current command and the actual three-phase current is inputted and the direct current link outputs a duty command so as to follow the command voltage. The method of claim 5,
Wherein the voltage controller is a proportional-integral controller.

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